A Posters

Tuesday, April 11, 2000 5:00 PM in Room Atlas Foyer

AP-1 Fatigue properties of a 4340 Steel Coated with a Colmonoy 88 Deposit Applied by HVOF
L. Hernández, F. Oliveira, C. Villalobos, J. Berríos, E.S. Puchi (Universidad Central de Venezuela)
The present investigation has been conducted in order to study the effect of a Colmonoy 88 deposit of 220 microns in thickness, on the fatigue properties of a quenched and tempered 4340 steel. Also, the effect of alumina blasting on the surface of the samples previous to the thermal spray process on the fatigue properties of the substrate has been evaluated. The coating was deposited by means of HVOF and the surface hardness was found to be approximately 676 VHN@sub300@. The tensile tests that were conducted on the coated samples showed a decrease in its static mechanical properties in comparison with the uncoated substrate, which is attributed to the early fracture of the deposit. The fatigue tests were carried out at 50 Hz. It has been determined that the alumina blasting gives rise to a reduction in the fatigue properties of the substrate as a result of the penetration of the alumina particles in the steel, giving rise to stress concentration sites that induce the nucleation of fatigue cracks. The fatigue performance of the substrate is reduced even more when the material is blasted and subsequently coated. The Basquin equations corresponding to the different conditions tested have been determined, in order to be used in design purposes. SEM studies were also carried out in order to characterize the fracture surfaces of the samples.
AP-2 Mechanical Properties of EB-PVD-Thermal Barrier Coatings by Nanoindentation
E. Lugscheider, K. Bobzin, A. Etzkorn (University of Technology Aachen, Materials Science Institute, Germany)
Ceramic thermal barrier coatings (TBCs) are used more than ever in the hot section of landbased gas turbines. The necessity of TBCs deposited on turbine blades is caused by a growing environmental consciousness and the need of an economical use of fossil fuels. To reach targets like high efficiency or a long life time of the gas turbine components, simulation and modelling of thermal barrier coating systems becomes more and more important. Introducing a model of a TBC system requires the knowledge of the characteristic values in the real material composite. In the present work EB-PVD zirconia thermal barrier coatings were examined by nanoindentation. Because of the microstructure of EB-PVD-coatings, the mechanical properties in the vertical and the horizontal direction are completely different. Therefore the indents were performed on cross sections and on the coatings surface itself. Because of the different behaviour at the grain boundaries and the grains, the measurement locations were statistically chosen. The measurement results were compared to SEM-analysis of the indents to illustrate the behaviour at grains and grain boundaries. The TBCs were deposited on NiAl intermetallic material at different substrate temperatures and different oxygen partial pressures during the deposition process. The change of this parameters caused different microstructures of the coatings, so the influence of the processing conditions on mechanical properties was examined.
AP-3 Performance of Chromium Nitride Based Coatings Under Plastic Processing Conditions
L. Cunha (Universidade do Minho, Portugal); M. Andritschky (University of Minho, Portugal); L. Rebouta (Universidade do Minho, Portugal); A. Zarychta (Silesian Technical University, Poland); A.S. Miranda, A. Cunha (Universidade do Minho, Portugal); K. Pischow (Surfcoat Oy., Finland); Z. Wang (Surfcoat Oy, Finland)
The processing of filled and reinforced plastics may cause abrasive wear on the surface of machine parts due to the high hardness of filler particles. The decomposition of additives may also cause corrosion. In addition, relatively high processing temperatures can strongly increase total wear of the surface of machine parts, dies and moulds. The deposition of ceramic hard coatings may increase lifetime of machine parts. Chromium nitride and multilayer titanium/chromium nitride based coatings were deposited onto steel substrates. The coatings were deposited by RF and DC magnetron sputtering. Mechanical properties like residual stress, hardness, Young's modulus, adhesion and wear rate were measured. Generally chromium nitride coatings are harder, have better adhesion, and lower wear rates but can be highly stressed. Multilayer coatings show significantly lower hardness and higher wear rates, but the residual stress levels were lower than -1 GPa. The corrosion performance was also studied by aqueous and high temperature corrosion tests. The corrosion behaviour of multilayer coatings was better than that one performed by chromium nitride coatings. Multilayer coatings may have the possibility of avoiding crevice corrosion attack on the substrate through pores and pinholes. Any defect that appears in a monolayer during the deposition process can be neutralised by the following monolayer. After these tests, the substrate/coating systems were tested under plastic processing conditions. The wear was measured and the wear tracks were analyzed by scanning electron microscopy. In general coatings with higher hardness, density, adhesion and corrosion performance show better wear resistance provoked by molten plastic.
AP-4 Characterization of Duplex Cr@sub 2@O@sub 3@/CrN Coatings for Wear Applications
D.Y. Wang, M.C. Chiu (National Chung Hsing University, Taiwan, ROC)
Chromium nitride (CrN) thin film deposited by PVD process has been used in metal forming applications for its superior wear performance. On exposure to high temperatures, CrN will form a dense and thin Cr@sub 2@O@sub 3@ oxide scale, which provides excellent wear resistance at elevated temperatures. In this study, we deposited a duplex Cr@sub 2@O@sub 3@/CrN coating, consisting of a thin Cr@sub 2@O@sub 3@ oxide film on top of a CrN coating using an unbalanced magnetron sputtering (UBM) technique. The pre-formed oxide scale provides an advantage of uniform Cr@sub 2@O@sub 3@ coverage in wear environments. For wear applications where high contact temperatures are anticipated, the duplex Cr@sub 2@O@sub 3@/CrN coating becomes promising in improving tribological properties, which results in enhanced wear life. Characterization of microstructure was conducted using x-ray diffraction (XRD) and scanning electron microscopy (SEM). Thermal properties of the duplex coatings were analyzed using thermogravimetric (TG) and differential thermal (DT) analyses. The tribological evaluation was performed using scratch tester and pin-on-disk tribotester.
AP-5 Modelling Microcracking Mechanism in Thermal Barrier Coatings
M. Lugovy, V. Slyunyaev (National Academy of Sciences of Ukraine); V. Teixeira (University of Minho, Portugal)
Multilayered coatings are commonly used as protective coatings for advanced power engineering applications to improve performance, e.g. thermal barrier coatings (TBC´s) deposited by Plasma Spraying techniques are currently applied on gas turbine blades and diesel engine components. Tensile residual stresses in the ceramic coating cause perpendicular microcracking (through-thickness cracks) while compressive stresses tend to promote microcrack propagation along the interface. Microcracking in the ceramic top coating will affect the thermo-mechanical integrity of the functional coated component. The coating failure mode by through-thickness cracking depends on the magnitude of tensile residual stress and on the relative strengths of coating. The Plasma Sprayed coating is in a tensile stress at working temperatures of 1273 and 1423 K. At high temperature the coating relaxes almost immediately, by microcracking, to a tensile stress level of about 140 MPa. After heating the top coating develops on cooling an higher compressive stress due to the relaxation processes at high temperature. Coating lifetime is limited by its susceptibility to failure upon high temperature use (isothermal or cyclic heat treatment). The model of failure proposed in this contribution can be applied to n-phase ceramic-based coatings. The attempt is made to solve a physical problem of failure description of a microinhomogeneous coating as stochastic process of the cracking of separate structural elements and damage accumulation. The particular modeling is executed for n = 1 (this corresponds to single-phase ceramic-based coating) under tension at high temperature. The above mentioned model is applied for the description of mechanical behaviour of single-phase ceramic-based coating with various statistical distributions of the grains sizes. An effective continuum conception and a statistical approach for description of the failure process is used. Characteristics of the individual structural elements and loading parameters are used as the initial data for calculation of volume fraction of the fractured structural element. According to energetic microcracking criterion, the more is structural elements size, the earlier is moment of its fracture .The failure process is found to have four main stages. The loading without microcracking and damage accumulation is first stage of the process. Second stage corresponds to a stable non-localized microcracking before stress maximum. Third stage is a stable localized microcracking after stress maximum. Fourth stage is a unstable (catastrophic) fracture. If the third or fourth stage of the stochastic failure process is reached, the magistral crack is formed in coating promoting to its delamination and spallation. The critical ratio of maximum grain size to average grain size is found to be in the coating. This ratio determines the dispersion of grain size distribution in coating. The fracture is total catastrophical (from first crack) when the dispersion of grain sizes is more than some critical value. The proposed model provides prediction on the dependence of the critical parameters of fracture for single-phase and multiphase ceramic-based coatings and the mechanical behavior of loaded structural elements. -